Wheels and casters are commonly used to reduce friction as heavy loads are transported on a frame or platform across a surface. When a source of pressurized air is available and the surface is relatively smooth and flat, air bearings can be used in place of wheels and casters. An air bearing is a flexible diaphragm with one or more small air exit holes that is sealed around its periphery and attached at its center to a lightweight backing plate. Pressurized air flows into the interior of the air bearing and inflates it. The air leaving the diaphragm through the exit holes first enters a plenum space under the diaphragm and then flows to the atmosphere. The continuously flowing film of air between the diaphragm and the surface allows friction to be virtually eliminated during transport.
Air bearings are made in many different sizes and shapes, including circular, square, and "racetrack." Air bearings are used with a supporting frame or platform. The combination of an air bearing and a frame is commonly known as an air bearing assembly. The frame typically contains two parallel plates separated by a small distance. The load rests on the upper plate while the lightweight backing plate of the air bearing is supported by the lower plate. Air bearings are used in a variety of air bearing assembly configurations. For example, air bearings are individually mounted to frames and used singly or, more commonly, in sets of three or more under heavy equipment. Air bearings are also used under moving platforms and rotating turntables. Air bearings can carry tremendous loads, for example, a round air bearing having a diameter of about 4 ft (1.2 m) can carry a load of about 40,000 lbs (18,200 kg). The air pressure in the diaphragm is a function of the load carried and does not depend on the inlet air pressure. The diaphragm will overinflate and may burst if the load capacity is exceeded.
A variety of operating conditions can cause pressure fluctuations to occur in the diaphragm of an air bearing. Rapid pressure fluctuations, in turn, can cause individual air bearings to bounce or "hop" and can cause a load resting on a set of air bearings to rock or sway. Common operating conditions that are believed to cause hopping and rocking include, but are not limited to, fluctuations in the pressurized air source itself, shifting of the load on the air bearing, and passing of the air bearing(s) over non-uniform surfaces. More particularly, if the surface over which the air bearing moves is not perfectly smooth, some of the exit holes may be temporarily blocked which, in turn, causes a non-uniform flow of air from the exit holes of the diaphragm. To eliminate or reduce hopping and rocking, most air bearings are used in conjunction with a damping chamber. A damping chamber is a sealed chamber with an orifice between it and the interior of the air bearing. The two-way air flow between the diaphragm and the damping chamber, together with the inherent compressibility of air, greatly reduces pressure fluctuations in the diaphragm.
Unfortunately, damping chambers add to the cost, weight, and manufacturing complexity of frames for air bearing assemblies. Damping chambers are sealed pressure vessels that are strong enough to withstand the pressure differential between the operating pressure of the air bearing and atmospheric pressure. Damping chambers are either purchased or fabricated by welding or other time-consuming and expensive process. They are often mounted on the frame of an air bearing assembly between the upper and lower plates. When added to the frame in this manner, the damping chamber makes little or no contribution to the frame's structural strength.
An air bearing assembly is disclosed in Burdick, U.S. Pat. No. 3,756,342, issued Sep. 4, 1973, which is incorporated by reference. The Burdick air bearing assembly contains a flexible diaphragm and a backing plate. The air bearing slides in and out from under a load carrying platform. The load carrying platform contains a sealed damping chamber which is presumably formed by welding. The damping chamber also contains a small orifice that communicates through a corresponding hole in the backing plate with the interior of the diaphragm. A gasket is placed between the orifice and the backing plate. The gasket is sealed by the deformation of the backing plate and, when the air bearing is pressurized, by the load.
It would be a major improvement if an air bearing assembly were provided that has an integral damping chamber that is easily and simply manufactured and whose seal is created by the pressurization of the air bearing.